The experiments described in this application are aimed at exploring the mechanisms underlying the SV40 transforming process. Specifically, my initial goal is to investigate the possibility of developing cell lines in which SV40 large and small T antigen synthesis can be efficiently induced and repressed by the simple addition or removal of a non-toxic inducer. I plan to take advantage of earlier work from this laboratory and that of Hu and Davidson (1987) which demonstrates that the regulatory elements of the E. Coli lac operon function efficiently in animal cells. In particular, it was shown that SV40 T and t antigen synthesis could be tightly regulated by these elements. Both in transient assays and, more recently here, in stable cell lines, the data indicate that the function of the SV40 early promoter can be reversibly and efficiently repressed by intact lac repressor, synthesized in the same cells. Induction was achieved by addition to the culture medium of the allolactose analog, IPTG; and repression was achieved by its removal. The ability to repeatedly induce and repress the synthesis of these two oncogene products could facilitate analyses of their specific roles in promoting the appearance of various aspects of a neoplastic phenotype in cultured cells and in supporting tumor growth In Vivo.